1. Field of the Invention
This invention relates to magnetic resonance imaging methods of and apparatus for obtaining morphological information and functional information, such as spectroscopy, of a subject under examination utilizing magnetic resonance (MR) phenomena.
2. Description of the Related Art
A method of obtaining a surface anatomy scan (SAS) image, which is one of MR images obtained by magnetic resonance imaging apparatus, was proposed by Katada et al in 1987. A patent application for the method was filed with the Japanese Patent Office on Sep. 17, 1987 (bearing filing No. 62-232949and titled "Magnetic Resonance Imaging Method").
Subsequent researches have confirmed that such SAS images have a very important significance in the field of neurosurgery. That is, SAS images clearly indicate cortices representing brain surface structures, including brain grooves, and the positions of diseased portions locally existing under cortices. This permits a diseased portion to be located with accuracy prior to surgical treatment for cerebral disease.
There are two SAS imaging techniques. The first technique, which requires that cerebrospinal fluid (CSF) be represented to make a sharp contrast to parenchyma, uses a spin-echo method in which the echo time TE and the pulse repetition time TR are set very long and obtains an image from a single, thick slice by the use of a surface coil. The other technique, which also requires that cerebrospinal fluid be represented to make a sharp contrast to parenchyma, uses a spin-echo method in which the echo time TE and the pulse repetition time TR are set very long, acquires images from multiple slices by the use of a head coil and performs weighted addition on the resultant images.
The above-described SAS imaging techniques require an imaging time of more than ten minutes. Thus, it is difficult to perform the SAS imaging in addition to the routine imaging. To clear such a difficulty, an SAS imaging technique intended to shorten imaging time is disclosed in Japanese Unexamined Patent Publication No.2 -200241. This technique is referred to as a 2-dimension-contrast enhanced-Fourier acquired steadystate (2D-CE-FAST) technique. The CE-FAST technique is among techniques which cause steady-state free precession (SSFP) of magnetic vectors. According to the CE-FAST pulse sequence, of free induction decay (FID) signals and echo signals generated in the steady state, only the echo signals are selectively acquired, thereby permitting CSF to be represented at high level in a short time. Note that the FID signals and the echo signals are all magnetic resonance signals.
Although, of course, the above 2D-CE-FAST technique has improved imaging time over the spin-echo SAS imaging technique, there is no denying the fact that its images are lower in image quality than spin-echo SAS images. This is because nuclear magnetic moments in the steady state are dephased with respect to one another in the direction of the thickness of a slice and thus lack of uniformity of magnetic resonance signals appears on an MR image.